Transporting services such as Internet Protocol TeleVision (IPTV) requires traffic engineering mechanisms for optimizing the use of resources, guaranteeing a quality of service appropriate to the service, and minimizing service downtime in the event of a fault. The Point-to-Multipoint Multi-Protocol Label Switching Traffic Engineering (P2MP MPLS-TE) technology exists for this purpose. It makes it possible to set up a point-to-multipoint tree in a Multi-Protocol Label Switching (MPLS) communications network with reservation of resources. The associated protocol, specified in the Internet Engineering Task Force (IETF) document RFC 4875, is an extension of the resource reservation (RSVP-TE) protocol and makes it possible to set up MPLS trees called P2MP LSP in which packets are routed explicitly. These trees replicate traffic from a node called the root node to a set of nodes called the leaf nodes that are thereafter responsible for routing the traffic to receivers. Explicit P2MP RSVP-TE routing makes it possible to set up trees that minimize bandwidth consumption and also to reserve resources and thus to provide a guaranteed quality of service when routing packets.
To minimize the impact of a fault on a service and in particular the impact on the television pictures of an IPTV service, it must be possible to re-route packets in less than about fifty milliseconds. The IETF P2MP MPLS-TE Fast ReRoute (FRR) mechanism (see draft-ietf-mpls-p2mp-te-bypass-01) is based on local back-up trees bypassing the protected element; it therefore makes it possible to guarantee the above-mentioned level of security in the event of a fault on a P2MP LSP link or in a transit node.
In contrast, it does not enable fast rerouting of packets in the event of a fault in a root node.
Below, references to a fault affecting a root node refer either to a fault in the node itself or a fault on a link supporting a branch coming from it.
A mechanism for protecting the root node is proposed in the IETF document draft-cao-mpls-te-p2mp-head-protection-01.txt. In the event of a fault affecting its primary root, the protection of a P2MP LSP, called the primary tree, is based on a back-up tree set up from a back-up root. The broadcast traffic is sent by a broadcast source to both the primary root and the back-up root. This back-up tree has as leaves one or more nodes downstream of the primary root on the primary LSP. To be more precise, these connecting nodes, known as merge nodes, belong to the primary tree and are positioned immediately downstream of the primary root on the various branches of the primary tree coming from the root. Thus once the back-up tree has been set up, it is possible to re-inject traffic coming from the back-up tree into the branches of the primary tree situated downstream of the merge nodes. In nominal mode, only the primary tree is active, i.e. only the primary root injects traffic into the network: the back-up tree is set up but inactive, i.e. the back-up root destroys broadcast traffic received from the broadcast source. If the back-up root detects a fault affecting the primary root, it activates the back-up tree and routes the broadcast traffic in the back-up tree. Thus only one copy of the traffic reaches the leaf nodes, both under nominal conditions and during a fault.
However, in the event of a fault affecting the root, this protection mechanism is based on the back-up root detecting the primary root fault. The back-up root may be at a great distance from the primary root and it is then very difficult to ensure detection of the fault in less than about fifty milliseconds. This also applies when using a fault detection protocol such as the Bidirectional Forwarding Detection (BFD) protocol specified in the IETF document draft-ietf-bfd-base-07.txt.
It is also very difficult to synchronize the two roots. If the primary and back-up roots are far apart, it is possible for the back-up root to misdiagnose a primary root fault, for example if there is a problem on a link between both roots. In this situation the back-up root activates the back-up tree when the primary tree is still active. The receivers receive duplicated traffic coming from the two roots, which in an IPTV service is equivalent to a fault: for example, in a Moving Picture Experts Group (MPEG) television service, duplication leads to loss of picture. Thus some of the advantage linked to fast traffic rerouting is lost.
There is therefore a requirement for a technique making it possible to guarantee traffic rerouting within a time period compatible with services requiring a guaranteed quality of service and without duplicating traffic in the event of a fault of the root node of a point-to-multipoint label switching tree.